Method and system for testing during operation of an open ring optical network

ABSTRACT

The method and system for testing during operation of an open ring network includes opening a ring of the network at the node. Traffic circulating on the ring is dropped in the node to a monitoring element prior to or at the opening of the ring in the node. At the monitoring element, a signal received from the ring is tested. During tested and while the ring is open, traffic continues to be communicated between each node on at least one of the ring or a second ring each connecting the nodes of the network.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to optical transport systems,and more particularly to a method and system for testing duringoperation of an open ring optical network.

BACKGROUND OF THE INVENTION

Telecommunications systems, cable television systems and datacommunication networks use optical networks to rapidly convey largeamounts of information between remote points. In an optical network,information is conveyed in the form of optical signals through opticalfibers. Optical fibers comprise thin strands of glass capable oftransmitting the signals over long distances with very low loss.

Optical networks often employ wavelength division multiplexing (WDM) ordense wavelength division multiplexing (DWDM) to increase transmissioncapacity. In WDM and DWDM networks, a number of optical channels arecarried in each fiber at disparate wavelengths. Network capacity isbased on the number of wavelengths, or channels, in each fiber and thebandwidth, or size of the channels. Arrayed waveguide gratings (AWGs),interleavers, and/or fiber gratings (FGs) are typically used to addand/or drop traffic at the multiplex and demultiplex network nodes.

SUMMARY OF THE INVENTION

The present invention provides a method and system for testing duringoperation of an open ring optical network. As a result, light paths,control channels, localized areas and/or replacement equipment may betested to locate faults and/or to confirm operation of equipment orlines in the network.

In accordance with one embodiment of the present invention, a method andsystem for testing during operation of an open ring network includesopening a ring of the network at a node. Traffic circulating on the ringis dropped in the node to a monitoring element prior to or at theopening of the ring in the node. At the monitoring element, a signalreceived from the ring is tested. During testing traffic continues to becommunicated between each node on at least one of the ring or a secondring each connecting the nodes of a network.

More specifically, in accordance with a particular embodiment of thepresent invention, the monitoring element may be an optical spectrumanalyzer, an element management system, or other suitable device. Inthis and other embodiments, the ring may be opened with a switch. Theswitch may be a 2×2 switch operable in a closed position to forwardtraffic along the ring and in a crossed position operable to directtraffic from the ring to the monitoring element.

Technical advantages of the present invention include providing nodeand/or component isolation, loopback and testing features for an openring network. In a particular embodiment, optical and/or electricloopbacks within the nodes may be configured so as to facilitate testingand/or insertion of new or replacement nodes, elements or components.Thus, the elements and functionality of the network can be tested aswell as unit operation and/or fiber connections. In addition, areas ofthe network may be localized from the in-service network for testingwhile maintaining full connectivity between nodes of the network.

It will be understood that the various embodiments of the presentinvention may include some, all, or none of the enumerated technicaladvantages. In addition, other technical advantages of the presentinvention may be readily apparent from the following figures,description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, wherein like numeralsrepresent like parts, in which:

FIG. 1 is a block diagram illustrating an optical network in accordancewith one embodiment of the present invention;

FIG. 2 is a block diagram illustrating details of the node of theoptical network of FIG. 1 in accordance with one embodiment of thepresent invention;

FIG. 3 is a block diagram illustrating details of an optical coupler inaccordance with one embodiment of the present invention;

FIG. 4 is a block diagram illustrating the optical splitter unit of atransport element of FIG. 2 in accordance with another embodiment of thepresent invention;

FIG. 5 is a block diagram illustrating details of the node of theoptical network of FIG. 1 in accordance with another embodiment of thepresent invention;

FIG. 6 is a flow diagram illustrating a method for passively adding anddropping traffic in an optical network in accordance with one embodimentof the present invention;

FIG. 7 is a flow diagram illustrating a method for protection switchingfor an open ring photonic network in accordance with one embodiment ofthe present invention;

FIG. 8 is a block diagram illustrating an optical network in accordancewith another embodiment of the present invention;

FIG. 9 is a block diagram illustrating details of the node of theoptical network of FIG. 8 in accordance with one embodiment of thepresent invention;

FIGS. 10A–C are block diagrams illustrating elements of the node of theoptical network of FIG. 8 in accordance with other embodiments of thepresent invention;

FIG. 11 is a block diagram illustrating the distributing element of FIG.9 in accordance with another embodiment of the present invention;

FIG. 12 is a block diagram illustrating the combining element of FIG. 9in accordance with another embodiment of the present invention;

FIG. 13 is a block diagram illustrating the open ring configuration andlight path flow of the optical network of FIG. 8 in accordance with oneembodiment of the present invention;

FIG. 14 is a block diagram illustrating the optical supervisory channel(OSC) flow in the optical network of FIG. 8 in accordance with oneembodiment of the present invention;

FIG. 15 is a block diagram illustrating protection switching and lightpath protection in the optical network of FIG. 8 in accordance with oneembodiment of the present invention;

FIG. 16 is a flow diagram illustrating a method for protection switchingfor the optical network of FIG. 8 in accordance with one embodiment ofthe present invention;

FIG. 17 is a block diagram illustrating OSC protection in the opticalnetwork of FIG. 8 in response to a line cut in accordance with oneembodiment of the present invention;

FIG. 18 is a flow diagram illustrating a method for OSC protectionswitching in the optical network of FIG. 8 in accordance with oneembodiment of the present invention;

FIG. 19 is a block diagram illustrating OSC protection in the opticalnetwork of FIG. 8 in response to an OSC equipment failure in accordancewith one embodiment of the present invention;

FIG. 20 is a block diagram illustrating loopback testing of a light pathin the optical network of FIG. 8 in accordance with one embodiment ofthe present invention;

FIG. 21 is a block diagram illustrating loopback testing of a light pathin the optical network of FIG. 8 in accordance with another embodimentof the present invention;

FIG. 22 is a block diagram illustrating localized area testing in theoptical network of FIG. 8 in accordance with one embodiment of thepresent invention;

FIG. 23 is a flow diagram illustrating a method for inserting a nodeinto the optical network of FIG. 8 in accordance with one embodiment ofthe present invention;

FIG. 24 is a block diagram illustrating the optical network of FIG. 8with passive nodes in accordance with one embodiment of the presentinvention;

FIG. 25 is a block diagram illustrating details of the passive node ofFIG. 24 in accordance with one embodiment of the present invention; and

FIG. 26 is a block diagram illustrating details of the passive node ofFIG. 24 in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an optical network 10 in accordance with oneembodiment of the present invention. In this embodiment, the network 10is an optical network in which a number of optical channels are carriedover a common path at disparate wavelengths. The network 10 may be awavelength division multiplexing (WDM), dense wavelength divisionmultiplexing (DWDM), or other suitable multi-channel network. Thenetwork 10 may be used in a short-haul metropolitan network, andlong-haul inter-city network or any other suitable network orcombination of networks.

Referring to FIG. 1, the network 10 includes a plurality of nodes 12, afirst fiber optic ring 14, and a second fiber optic ring 16. Opticalinformation signals are transmitted in different directions on the rings14 and 16 to provide fault tolerance. Thus each node both transmitstraffic to and receives traffic from each neighboring node. As usedherein, the term “each” means every one of at least a subset of theidentified items. The optical signals have at least one characteristicmodulated to encode audio, video, textual, real-time, non-real-timeand/or other suitable data. Modulation may be based on phase shiftkeying (PSK), intensity modulation (IM) and other suitablemethodologies.

In the illustrated embodiment, the first ring 14 is a clockwise ring inwhich traffic is transmitted in a clockwise direction. The second ring16 is a counterclockwise ring in which traffic is transmitted in acounterclockwise direction. Span A comprises the portion of theclockwise ring 14 and counterclockwise ring 16 between node 18 and node20. Span B comprises the portion of the clockwise ring 14 and thecounterclockwise ring 16 between node 20 and node 22. Span C comprisesthe portion of the clockwise ring 14 and the counterclockwise ring 16between nodes 22 and 24. Span D comprises the portion of the clockwisering 14 and the counterclockwise ring 16 between node 24 and node 18.

The nodes 12 are operable to add and drop traffic to and from the rings14 and 16. At each node 12, traffic received from local clients is addedto the rings 14 and 16 while traffic destined for local clients isdropped. Traffic may be added to the rings 14 and 16 by inserting thetraffic channels or otherwise combining signals of the channels into atransport signal of which at least a portion is transmitted on one orboth rings 14 and 16. Traffic may be dropped from the rings 14 and 16 bymaking the traffic available for transmission to the local clients.Thus, traffic may be dropped and yet continue to circulate on a ring 14and 16. In a particular embodiment, traffic is passively added to anddropped from the rings 14 and 16. “Passive” in this context means theadding or dropping of channels without power, electricity, and/or movingparts. An active device would thus use power, electricity or movingparts to perform work. In a particular embodiment, traffic may bepassively added to and/or dropped from the ring 14 and 16 bysplitting/combining, which is without multiplexing/demultiplexing, inthe transport rings and/or separating parts of a signal in the ring.

In one embodiment, the nodes 12 are further operable to multiplex datafrom clients for adding to the rings 14 and 16 and to demultiplexchannels of data from the rings 14 and 16 for clients. In thisembodiment, the nodes 12 may also perform optical to electricalconversion of the signals received from and sent to the clients.

In addition, as described in more detail below, rings 14 and 16 eachhave termini in one of the nodes 12, such that the rings 14 and 16 are“open” rings. That is, the rings 14 and 16 do not form a continuoustransmission path around network 10 such that traffic does not continueand/or include an obstruction on a ring past a full circuit of thenetwork 10. The opening in the rings 14 and 16 terminates, and thusremoves channels at the terminal points. Thus, after traffic of achannel is transmitted to each node 12 in the clockwise and/orcounterclockwise rings 14 and 16 by the combined nodes 12, the trafficis removed from the rings 14 and 16. This prevents interference of eachchannel with itself.

In a particular embodiment and as described further below, signalinformation such as wavelengths, power and quality parameters aremonitored in the nodes 12 and/or by a control system. Based on thisinformation, the network 10 is alert to line cuts and other faults andis able to perform protection switching. Thus, the nodes 12 provide forcircuit protection in the event of a line cut in one or both of therings 14 and 16.

Total lambda of the network 10 may be divided and assigned to each node12 depending on the local or other traffic of the nodes 12. For anembodiment in which the total lambda is forty and total number of nodes12 is four and the node traffic is even in each node 12, then ten lambdamay be assigned to each node 12. If each lambda is modulated by 10 Gb/sdata-rate, each note can send 100 Gb/s (10 Gb/s×10 lambda) to all nodesin the network 10. For a DWDM system, the lambda may be between 1530 nmand 1565 nm. The channel spacing may be 100 GHz or 0.8 nm, but may besuitably varied. In addition, channel spacing is flexible in the rings14 and 16 and the node elements on the rings 14 and 16 need not beconfigured with channel spacing. Instead, for example, channel spacingmay be set up by add/drop receivers and transmitters that communicatewith and/or are coupled to the clients. The rings 14 and 16 add, dropand communicate traffic independently of and/or regardless of thechannel spacing of the traffic.

FIG. 2 illustrates details of the node 12 in accordance with oneembodiment of the present invention. In this embodiment, traffic ispassively added to and dropped from rings 14 and 16 by optical couplersor other suitable optical splitters. An optical splitter is any deviceoperable to combine or otherwise passively generate a combined opticalsignal based on two or more optical signals without multiplexing and/orto split or divide an optical signal into discrete optical signals orotherwise passively generate discrete optical signals based on theoptical signal without demultiplexing. The discrete signals may besimilar or identical in form and/or content. For example, the discretesignals may be identical in content and identical or substantiallysimilar in energy, may be identical in content and differ substantiallyin energy, or may differ slightly or otherwise in content.

Referring to FIG. 2, the node 12 comprises a first, or counterclockwisetransport element 30, a second, or clockwise transport element 32, acombining element 34 and a distributing element 36. The transportelements 30 and 32 each passively add and drop traffic to and from therings 14 and 16 without multiplexing or demultiplexing or signals on thering and/or provide other interaction of the node 12 with the ring. Thecombining element 34 generates the local add signal passively orotherwise. The distributing element 36 distributes the drop signals intodiscrete signals for recovery of local drop traffic passively orotherwise. In a particular embodiment, the transport, combining anddistributing elements 30, 32, 34 and 36 may each be implemented as adiscrete card and interconnected through a backplane of a card shelf ofthe node 12. In addition, functionality of an element itself may bedistributed across a plurality of discrete cards. In this way, the node12 is modular, upgradeable, and provides a pay-as-you-grow architecture.

Each transport element 30 and 32 is connected or otherwise coupled tothe corresponding ring 14 or 16 to add and drop traffic to and from theconnected ring 14 or 16. Components may be coupled by direct, indirector other suitable connection or association. In one embodiment, thetransport elements 30 and 32 each include an ingress amplifier 40, aring protection switch 41, an optical splitter 42, and an egressamplifier 44. In the illustrated embodiment, the elements of the node 12and devices in the elements are connected with optical fiberconnections, however, other embodiments may be implemented in part orotherwise with planar wave guide circuits and/or free space optics.

The ingress and egress amplifiers 40 and 44 may be erbium-doped fiberamplifier (EDFAs) or other suitable amplifiers capable of receiving andamplifying an optical signal. The output of the amplifier may be, forexample, 5 dBm. As the span loss of clockwise ring 14 usually differsfrom the span loss of counterclockwise ring 16, egress amplifier 40 mayuse an automatic level control (ALC) function with wide inputdynamic-range. Hence, the ingress amplifier 40 may deploy automatic gaincontrol (AGC) to realize gain-flatness against input power variation aswell as variable optical attenuators (VOAs) to realize ALC function. Ina particular embodiment, the amplifier 40 may be gain variableamplifiers, such as, for example, as described in U.S. Pat. No.6,055,092.

The ring protection switch 41 is a two position or other suitable switchor device operable to selecting open or close the connected ring at thenode 12. The optical splitter units 42 may each be an optical fibercoupler or other optical splitter operable to combine and/or split anoptical signal. Details of the optical coupler are described in moredetail in connection with FIG. 3.

In operation of the transport elements, the ingress amplifier 40receives an ingress transport signal from the connected ring 14 or 16and amplifies the signal. Protection switches 41 allow the network 10 toreconfigure traffic flow in response to a line cut or other fault toprovide fault tolerance. The amplified signal is passed to the opticalcoupler 42. The optical coupler 42 combines the amplified ingress signalwith a local add signal from the combining element 34 to generate acombined signal. The optical coupler 42 further splits the combinedsignal into an egress transport signal for transmission on the connectedring 14 or 16 and a local drop signal from the ring 14 or 16. The localdrop signal is passed to the distributing element 36 for processing. Inthis way, for example, traffic is passively added to and dropped fromthe ring 14 or 16 in the node 12.

The combining element 34 includes a multiplexer 46 and an opticalcoupler 48. The multiplexer 46 multiplexes a plurality of local signalsto generate the local add signal. The optical coupler 48 splits thelocal add signal into two generally content-identical local add signals,one of which is passed to each transport element 30 and 32. Themultiplexer may be an arrayed wave guide (AWG).

The distributing element 36 includes a demultiplexer 50 and an opticalcoupler 52. The optical coupler 52 combines the local drop signals ofeach ring 14 and 16 provided by the transport elements 30 and 32 togenerate a local drop signal for the node 12. The local drop signal isdemultiplexed by demultiplexer 50 after which the discrete signals maybe filtered and distributed.

A transponder 56 may be connected between the node 12 and a client orset of clients 58. In the metro environment, the client 58 may be acorporate campus, industrial complex, large building, set of buildings,a city block, a neighborhood and the like. For the long haulenvironment, the client may be a town, small city or geographic region.

The transponder 56 includes optical receivers 60 and optical senders 62operable to receive an optical signal from the client 58, change thewavelength of the signal as necessary to avoid interference in network10, and send the optical signal to the multiplexer 46 of the combiningelement 34 over an optical link. The transponder 56 also includesoptical receiver 64 and optical sender 66 operable to receive a selecteddemultiplexed channel from the demultiplexer 50 of the distributorelement 36 over an optical link, change the wavelength as necessary toavoid interference in the client's network, and send the optical signalto the client 58. In changing the wavelength, the transponder 56 mayconvert the signal from an optical format to a non-optical format andback into the optical format.

Use of the transponder 56 allows the network 10 and client network toindependently set wavelengths for traffic flow. Client 58 may directlyconnect to the node 12 without transponder 56 when client 57 has asuitable interface to the node 12. To enhance flexibility, the opticalreceivers 60 and 64 may include tunable filters and the optical senders62 and 64 may include wavelength tunable filters while the opticalsenders 62 and 66 include wavelength tunable lasers. In this embodiment,a light path may be established between two nodes 12 by setting a laserof one of one of the optical senders in the transmitting node to aspecified frequency and correspondingly setting to the specifiedfrequency a filter of an optical receiver in the receiving node. Noother configuration is necessary in the network 10 as the trafficchannel may be passively combined with and separated from other trafficand is passively added to and dropped from the rings 14 and 16. It willbe understood that optical senders with fixed lasers and opticalreceivers with fixed filters may be used in connection with the presentinvention and that non-passive combining and distributing elements 36and 34 may also be used.

FIG. 3 illustrates details of an optical coupler 70 in accordance withone embodiment of the present invention. In the embodiment, the opticalcoupler 70 is a fiber coupler with two inputs and two outputs. Theoptical coupler 70 may in other embodiments be combined in whole or partwith a waveguide circuit and/or free space optics. It will be understoodthat the coupler 70 may include one or any number of any suitable inputsand outputs and that the coupler 70 may comprise a greater number ofinputs than outputs or a greater number of outputs than inputs.

Referring to FIG. 3, the optical coupler 70 comprises a cover frame 72,first entry segment 74, second entry segment 76, first exit segment 78,and second exit segment 80.

First entry segment 74 and first exit segment 78 comprise a firstcontinuous optical fiber. Second entry segment 76 and second exitsegment 80 comprise a second continuous optical fiber. Outside of themain body 72, segments 74, 76, 78, and 80 may comprise a jacket, acladding, and a core fiber. Inside the cover frame 72, the jacket andcladding may be removed and the core fibers twisted, fused, or coupledtogether to allow the transfer of optical signals and/or energy of thesignals between and among the first and second continuous opticalfibers. In this way, the optical coupler 70 passively combines opticalsignals arriving from entry segments 74 and 76 and passively splits andforwards the combined signal via exit segments 78 and 80. A plurality ofsignals may be combined and the combined signal split by combining andthereafter splitting the combined signal or by simultaneously combiningand splitting the signals by transferring energy between fibers. In thelater case, the optional coupler 70 includes intermediate signals.

The optical coupler 70 provides flexible channel-spacing with norestrictions concerning channel-spacing in the main streamline. Thecoupler 70 may split the signal into two copies with substantially equalpower. “Substantially equal” in this context means ±25%. The coupler mayhave a directivity of over 55 dB. Wavelength dependence on the insertionloss may be less than about 0.5 dB over 100 nm. The insertion loss for a50/50 coupler may be less than about 3.5 dB.

FIG. 4 illustrates the optical splitter unit 42 in accordance withanother embodiment of the present invention. In this embodiment, a pairof optical couplers 92 and 94 are used in each transport element 30 and32 of node 12. Thus, the combining and splitting of signals may beperformed by a single coupler with integrated optical combiner andsplitter elements or a plurality of couplers each having one or aportion of the combiner or splitter elements. Although the dual couplerarrangement increases the total number of couplers in the transportelements 30 and 32, the two-coupler arrangement may reduce channelinterference by dropping local traffic from a ring 14 or 16 beforeadding the local traffic to the ring 14 or 16.

Referring to FIG. 4, the first coupler 94 is an optical splitter elementthat splits an ingress transport signal 96 from the ingress amplifier 40and ring switch 41 into a pass through transport signal 98 and a localdrop signal 100. The local drop signal 100 is forwarded to the opticalcoupler 52 of the distributor element 36. A transport signal is a signaltransmitted on one or more of the rings 14 or 16, a passthrough signalforwarded by one coupler of a transport element to another coupler ofthe element, an intermediate signal of a coupler of a transport element30 or 32 and/or other non-add or non-drop signal within a process by atransport element 30 or 32.

The second optical coupler 94 is an optical combiner element thatcombines the pass through transport signal 98 and a local add signal 102from the optical coupler 48 of the combining element 34. In combiningthe pass though transport signal 98 with the local add signal 102, thesecond optical coupler 94 generates an egress transport signal 104. Theegress transport signal 104 is amplified by egress amplifier 44 fortransmission on the connected ring 14 or 16.

FIG. 5 illustrates details of node 12 in accordance with anotherembodiment of the present invention. In this embodiment, the node 12provides the add/drop features of the node 12 using separate add anddrop couplers as described in connection with FIG. 4.

Referring to FIG. 5, the node 12 includes a counterclockwise transportelement 110, clockwise transport element 112, a combining element 114,and a distributing element 116. The transport elements 110 and 112 areeach connected to a corresponding one of the rings 14 or 16 to add anddrop traffic to and from the connected ring 14 or 16.

The transport elements 110 and 112 each include an ingress amplifier120, a drop optical coupler 122, an optical ring switch 124, an addcoupler 126 and an egress amplifier 128. The ingress and egressamplifiers 120 and 128 may be EDFA or other suitable amplifiers. Thedrop optical coupler 122 splits an incoming signal into two outgoingsignals. The optical add coupler 124 combines two incoming signals intoan outgoing signal. The optical couplers 122 and 124 may be the opticalcoupler 70 modified with a single input or a single output.

In one embodiment, the ring switch 124 is a two position switch operableto open or close the corresponding ring 14 or 16. It will be understoodthat other suitable switches may be used. For example, as described inmore detail below, two-by-two switches may be used to support loopback,localized and other testing in addition to providing fault tolerance forthe network 10. The switches 124 may be controlled by a networkmanagement system (NMS) for network 10 or other suitable control system.

In operation of the transport elements 110 and 112, the ingressamplifier 120 receives an ingress transport signal from the connectedring and amplifies the signal. If the ring switch 124 is open, theamplified signal is terminated. Alternatively, if the ring switch 124 isclosed, the amplified signal is forwarded to the drop coupler 122. Thedrop coupler 122 splits the amplified ingress signal into a passthroughsignal and a local drop signal. The local drop signal is forwarded tothe distributing element 116. The passthrough signal is passed to theadd coupler 126. The add coupler 126 combines any passthrough signalwith any local add signal from the combining element 114 to generate anegress transport signal. The egress transport signal is amplified byegress amplifier 128 for transmission on the connected ring 14 or 16.Thus, traffic is passively added and dropped in the node 12.

The combining element 114 includes a multiplexer 130 and an opticalcoupler 132 for adding signals to each of the transport elements 110 and112. The multiplexer 130 multiplexes a plurality of local signals togenerate a local add signal. The optical coupler 132 splits the localadd signal into two generally content-identical add signals, one ofwhich is allocated to each transport element 110 and 112.

The distributing element 116 includes a demultiplexer 136, an opticalcoupler 138. The optical coupler 138 combines the local drop signals togenerate a local drop signal for the node 12. The local drop signal isdemultiplexed by the demultiplexer 136. In a particular embodiment, thetraffic may be transmitted to and received from a client through atransponder as previously described in connection with the node 12. Aclient may directly connect to the distributing element 116 withouttransponder 56 when the client has a suitable interface to the element116.

An NMS or other control system may control the ring switches 41 of FIG.2 or ring switches 124 of FIG. 5 directly or indirectly to provideprotection switching. In operation, one switch is open in each ring 14and 16 such that the rings 16 and 14 have termini at a node 12 so as tobe “open”. The openings in the rings are aligned with each other orotherwise correspond to each other such that they occur in a samesection of the rings 14 and 16. The same section may be between add anddrop couplers in a node and/or between optical splitter units ofneighboring nodes. The rings 14 and 16 are twin rings when the openingrings correspond to each other. The open rings prevent any traffic fromcirculating beyond a complete circuit of a ring 14 or 16 and thusinterfering with later transmitted traffic on the same channel.

In the event of a line cut and/or other openings of rings 14 and 16 inspan A, B, C, or D of network 10, ring switches 41 of the node 12 ofFIG. 2 or 124 of the node of FIG. 5 that bound the fault may be switchedto the “open” position and the previously open switches may be switchedto the “closed” position allowing signal traffic to pass through theprevious opening on rings 14 and 16. Thus, the switches are selectivelyclosable to provide protection switching. In this way, a line cut doesnot result in the isolation of a node 12 from the other nodes in thenetwork 10. Thus, if rings 14 and 16 are elsewhere open (as in a linecut), they can be closed in the other nodes 12. In the event of a linecut in a span of only one of ring 14 or 16, the span of the non cut ringmay be opened to simulate a cut at a corresponding point in the un-cutring. Corresponding point in this context means a point in one ringcorresponding to the same span as a defined point in the other ring. Byopening the un-cut ring at the corresponding point as the cut in the cutring, both rings are opened in the same span or section.

The above-described opening of the ring may be accomplished using anamplifier or other transmission device operable to selectively pass asignal through an optical fiber or effectively terminate the signal. Inone embodiment, the signal is effectively terminated when the signaldoes not interfere with new traffic being added to the ring. Furtherdetails of protection switching are described in connection with FIG. 7.

FIG. 6 is a flow diagram illustrating a method for adding and droppingtraffic channels in a flexible optical network in accordance with oneembodiment of the present invention. The flexible optical network may bethe open ring network of FIG. 1 or other suitable passive network.

Referring to FIG. 6, the method begins at step 160 where local channelsare received from a client. As previously described, the local channelsmay first be processed by a transponder to prevent inter networkwavelength conflicts and may be multiplexed by a multiplexer within thenode 12.

At step 162, the local channels are added to a pass-through portion ofan ingress signal from a ring 14 or 16 to form an egress signal. Aspreviously described, local channels may be added to the pass-throughportion of the ingress signal by an optical coupler or other suitableoptical splitter. Proceeding to step 164, the egress signal is amplifiedfor transmission on the ring 14 or 16. The signal may be amplified by anEDFA or other suitable amplifier.

Returning to the ingress signal, at step 166 the signal is obtained fromthe ring 14 or 16. At step 168, the ingress signal is amplified. Aspreviously described, the amplification may be by EDFA or other suitableamplifier.

Proceeding to step 170, the ingress signal is split into thepass-through portion and a drop portion. As previously described, theingress signal may be split by the optical coupler or other suitablepassive splitter. At step 172, locally-destined channels are retrievedfrom the drop portion of the combined signal. As previously described,the drop portion may be demultiplexed by a demultiplexer within the node12 or hub 18, allowing for the selection of channels for receipt by theclient. At step 174, the locally-destined channels are forwarded to theclient.

FIG. 7 is a flow diagram illustrating a method for protection switchingof an open ring photonic network in accordance with one embodiment ofthe present invention. In this embodiment, NMS in the network 10communicates with the nodes 12 to provide protection switching.

Referring to FIG. 7, the method begins at step 180 where the NMS detectsa loss of signal (LOS) at a node 12 in one of the rings 14 or 16indicating a fiber cut or other similar type of failure in one of thespans connecting the nodes 12 of the clockwise and/or counterclockwisering 14 and 16.

At step 182, the un-cut ring in the span is opened. This may beaccomplished by, in one embodiment, turning off the amplifiers adjacentto or that feed into the cut and also amplifiers at a correspondingpoint in the un-cut ring. In this way, a line cut in both rings 14 and16 at the same span is simulated and the rings 14 and 16 remain openafter the cut or failure is repaired. In accordance with this embodimentof the present invention, the protection switching may be accomplishedin less than 50 milliseconds.

Proceeding to step 184, after both rings 14 and 16 are opened in thespan of the fiber cut, previously open switches 41 or 124 in the nodes12 are switched to the “on” position, allowing the optical signal topass through the previous openings. In this way, ring integrity ismaintained and node 12 isolation is prevented.

FIG. 8 illustrates an optical network 200 in accordance with anotherembodiment of the present invention. In this embodiment, the network 200may include a plurality of nodes 201 with protection switching featuresintegrated into the node design. As a result, only a single type of nodewith a same or substantially similar element configuration need be used.

Referring to FIG. 8, the network 200 includes a first fiber optic ring202 and a second fiber optic ring 204 connecting nodes 206, 208, 210,and 212. As with network 10, network 200 is an optical network in whicha number of optical channels are carried over a common path at disparatewavelengths. The network 200 may be a wavelength division multiplexing(WDM), dense wavelength division multiplexing (DWDM), or other suitablemulti-channel network. The network 200 may be used in a short-haulmetropolitan network, and long-haul inter-city network or any othersuitable network or combination of networks.

In network 200, optical information signals are transmitted in differentdirections on the rings 202 and 204 to provide fault tolerance. Theoptical signals have at least one characteristic modulated to encodeaudio, video, textual, real-time, non-real-time and/or other suitabledata. Modulation may be based on phase shift keying (PSK), intensitymodulation (IM) and other suitable methodologies.

In the illustrated embodiment, the first ring 202 is a clockwise ring inwhich traffic is transmitted in a clockwise direction. The second ring204 is a counterclockwise ring in which traffic is transmitted in acounterclockwise direction. The nodes 201 are each operable to add anddrop traffic to and from the rings 202 and 204. In particular, each node201 receives traffic from local clients and adds that traffic to therings 202 and 204. At the same time, each node 201 receives traffic fromthe rings 202 and 204 and drops traffic destined for the local clients.In adding and dropping traffic, the nodes 201 may multiplex data fromclients for transmittal in the rings 202 and 204 and may demultiplexchannels of data from the rings 202 and 204 for clients.

As previously described in connection with network 10, traffic may beadded to the rings 202 and 204 by inserting the traffic channels orotherwise combining signals of the channels into a transport signal ofwhich at least a portion is transmitted on a ring. Traffic may bedropped by making the traffic available for transmission to the localclients. Thus, traffic may be dropped and yet continue to circulate on aring.

In a particular embodiment, traffic is passively added to and passivelydropped from the rings 202 and 204. In this embodiment, channel spacingis flexible in the rings 202 and 204 and the node elements on the rings202 and 204 need not be configured with channel spacing. Thus, channelspacing may be set by and/or at the add/drop receivers and senders ofthe nodes 201 coupled to the client. The transport elements of the nodes201 communicate the received traffic on the rings 202 and 204 regardlessof the channel spacing of the traffic.

Each ring 202 and 204 has a terminating point such that the rings 202and 204 are “open” rings. The opening in the rings 202 and 204 may be aphysical opening, an open, crossed, or other non-closed switch, adeactivated transmission device or other obstruction operable tocompletely or effectively terminate, and thus remove channels from therings 202 and 204 at the terminal points such that interference of eachchannel with itself due to recirculation is prevented or minimized suchthat the channels may be received and decoded within normal operatinglimits.

In one embodiment, the rings 202 and 204 are open, and thus terminate,in the nodes 201. In a particular embodiment, the rings 202 and 204 mayterminate in neighboring nodes 201 at corresponding points along therings 202 and 204. Terminal points in the rings 202 and 204 may becorresponding when, for example, they are between add and/or dropdevices of two neighboring nodes or when similarly positioned within asame node. Further details regarding the open ring configuration aredescribed below in reference to FIG. 13.

FIG. 9 illustrates details of the node 201 in accordance with oneembodiment of the present invention. In this embodiment, opticalsupervisory channel (OSC) traffic is transmitted in an external bandseparate from the revenue-generating traffic. In a particularembodiment, the OSC signal is transmitted at a wavelength of 1510nanometers (nm).

Referring to FIG. 9, the node 201 comprises counterclockwise transportelement 220, clockwise transport element 222, distributing element 224,combining element 226, and managing element 228. In one embodiment, theelements 220, 222, 224, 226 and 228 as well as components within theelements may be interconnected with optical fiber links. In otherembodiments, the components may be implemented in part or otherwise withplanar waveguide circuits and/or free space optics. In addition, asdescribed in connection with nodes 12, the elements of node 201 may eachbe implemented as one or more discrete cards within a card shelf of thenode 201. Exemplary connectors 230 for a card shelf embodiment areillustrated by FIG. 9. The connectors 230 may allow efficient and costeffective replacement of failed components. It will be understood thatadditional, different and/or other connectors may be provided as part ofthe node 201.

Transport elements 220 and 222 may each comprise passive couplers orother suitable optical splitters 70, ring switch 214, amplifier 215, andOSC filters 216. Optical splitters 70 may comprise splitters 70 or othersuitable passive device. Ring switch 214 may be a 2×2 or other switchoperable to selectively open the connected ring 202 or 204. In the 2×2embodiment, the switch 214 includes a “cross” or open position and a“through” or closed position. The cross position may allow for loopback,localized and other signal testing. The open position allows the ringopenings in the nodes 201 to be selectively reconfigured to provideprotection switching.

Amplifier 215 may comprise an EDFA or other suitable amplifier. In oneembodiment, the amplifier is a preamplifier and may be selectivelydeactivated to open a connected ring 202 or 204 to provide protectionswitching in the event of failure of the adjacent switch 214. Becausethe span loss of clockwise ring 202 usually differs from the span lossof counterclockwise ring 204, the amplifier 215 may use an ALC functionwith wide input dynamic-range. Hence, the amplifier 215 may deploy AGCto realize gain-flatness against input power variation as well as ALC byinternal VOA. The preamplifier 215 and the switch 214 are disposed inthe transport elements 220 and 222 inside of the OSC filters 216 andbetween the ingress OSC filter 216 and the add/drop couplers 70. Thus,the OSC signal may be recovered regardless of the position of switch 214or operation of preamplifier 215. OSC filters 216 may comprise thin filmtype, fiber grating or other suitable type filters.

In the specific embodiment of FIG. 9, counterclockwise transport element220 includes a passive optical splitter set having a counterclockwisedrop coupler 232 and a counterclockwise add coupler 234. Thecounterclockwise transport element 220 further includes OSC filters 294and 298 at the ingress and egress edges, counterclockwise amplifier 240between the ingress OSC filter 294 and drop coupler 232 andcounterclockwise ring switch 244 between amplifier 240 and drop coupler232. Thus, the switch 244 in this embodiment is on the ingress side ofthe transport element and/or drop coupler. The counterclockwisetransport element 220 may also include a dispersion compensation fiber(DCF) segment 245 to provide dispersion compensation. In one embodiment,DCF segment 245 may be included where the network 200 operates at ratesat or above 2.5 G, if the circumference of the ring is over 40kilometers, or depending on the length of the span to the previous node.For example, dispersion compensation may be used when 10 Gb/s signaltravels over 40 kilometers of 1.3 micrometer zero-dispersion single modefiber.

Clockwise transport element 222 includes a passive optical splitter setincluding clockwise add coupler 236 and clockwise drop coupler 238.Clockwise transport element 222 further includes OSC filters 296 and300, clockwise amplifier 242, and clockwise ring switch 246. OSC filters296 and 300 are disposed at the ingress and egress edges of theclockwise transport element 222. The clockwise amplifier 242 is disposedbetween the ingress OSC filter 300 and the drop coupler 238 while theclockwise ring switch 246 is disposed between the amplifier 242 and thedrop coupler 238. Thus, the switch 246 in this embodiment is on theingress side of the transport element and/or drop coupler. The clockwisetransport element 222 may also include a DCF segment 235 to providedispersion compensation depending, as previously discussed, on the datatransport rate and/or the length of the span to the previous node or thecircumference of the ring.

Distributing element 224 may comprise a plurality of distributingamplifiers 315. In this embodiment, the distributing element 224 maycomprise a drop coupler 310 feeding into the distributing amplifiers 315which each include an amplifier and an optical splitter. For example, afirst distributing amplifier 315 may include amplifier 316 and opticalsplitter 320 while a second distributing amplifier 315 may includeamplifier 316 and splitter 322. The amplifiers 316 and 318 may compriseEDFAs or other suitable amplifiers. Splitters 320 and 322 may comprisesplitters with one optical fiber ingress lead and a plurality of opticalfiber drop leads 314. The drop leads 314 may be connected to one or moretunable filters 266 which in turn may be connected to one or morebroadband optical receivers 268.

Combining element 226 may be a combining amplifier and may comprise asplitter 324 with a plurality of optical fiber add leads 312 which maybe connected to one or more add optical senders 270 associated with aclient. Splitter 324 further comprises two optical fiber egress leadswhich feed into amplifiers 326 and 328. Amplifiers 326 and 328 maycomprise EDFAs or other suitable amplifiers.

Managing element 228 may comprise OSC senders 272 and 281, OSCinterfaces 274 and 280, OSC receivers 276 and 278, and an elementmanagement system (EMS) 290. Each OSC sender, OSC interface and OSCreceiver set forms an OSC unit for one of the rings 202 or 204 in thenode 201. The OSC units receive and transmit OSC signals for the EMS290. The EMS 290 may be communicably connected to a network managementsystem (NMS) 292. NMS may reside within node 201, in a different node,or external to all of the nodes 201.

EMS 290, NMS 292 and/or other elements or parts of node 201 or network200 may comprise logic encoded in media for performing network and/ornode monitoring, failure detection, protection switching and loopback orlocalized testing functionality of the network 200. Logic may comprisesoftware encoded in a disk or other computer-readable medium and/orinstructions encoded in an application specific integrated circuit(ASIC), field programmable gate array (FPGA), or other processor orhardware. It will be understood that functionality of EMS 290 and/or NMS292 may be performed by other components of the network 200 and/or beotherwise distributed or centralized. For example, operation of NMS 292may be distributed to the EMS of nodes 201 and the NMS omitted.Similarly, the OSC units may communicate directly with NMS 292 and EMS290 omitted.

The node 201 further comprises counterclockwise add fiber segment 302,counterclockwise drop fiber segment 304, clockwise add fiber segment306, clockwise drop fiber segment 308, OSC fiber segments 282, 284, 286,and 288, and optical spectrum analyzer (OSA) connectors 250, 254, 256,and 258. The OSA connectors may be angled connectors to avoidreflection. Test signal may sometimes be fed into the network fromconnectors 248 and 252. As previously described, a plurality of passivephysical contact connectors 230 may be included where appropriate so asto communicably connect the various elements of node 201.

In operation, the transport elements 220 and 222 are operable topassively add local traffic to the rings 202 and 204 and to passivelydrop at least local traffic from the rings 202 and 204. The transportelements 220 and 222 may further be operable to passively add and dropthe OSC signal to and from the rings 202 and 204. More specifically, inthe counterclockwise direction, OSC filter 294 processes an ingressoptical signal from counterclockwise ring 204. OSC filter 294 filtersOSC signal from the optical signal and forwards the OSC signal to theOSC interface 274 via fiber segment 282 and OSC receiver 276. OSC filter294 also forwards or lets pass the remaining transport optical signal toamplifier 240. By placing the OSC filter 294 outside of the ring switch244, the node 201 is able to recover the OSC signal regardless of theposition of the ring switch 244.

Amplifier 240 amplifies the signal and forwards the signal to ringswitch 244. Ring switch 244 is selectively operable to transmit theoptical signal to coupler 232 when the ring switch 244 is set to thethrough (closed) setting, or to transmit the optical signal to OSAconnector 250 when the ring switch 244 is set to the cross (open)setting. Further details regarding the OSA connectors are describedbelow.

If ring switch 244 is set in the cross position, the optical signal isnot transmitted to couplers 232 and 234, the ring 204 is open at thenode 201, and dropping of traffic from the ring 204 does not occur atnode 201. However, adding of traffic at node 201 occurs and the addedtraffic flows to the next node in the ring 204. If the ring switch 244is set in the through position, the optical signal is forwarded tocouplers 232 and 234 and adding and dropping of traffic to and from thering 204 may occur at node 201.

Coupler 232 passively splits the signal from switch 244 into twogenerally identical signals. A passthrough signal is forwarded tocoupler 234 while a drop signal is forwarded to distributing element 224via segment 304. The signals may be substantially identical in contentand/or energy. Coupler 234 passively combines the passthrough signalfrom coupler 232 and an add signal comprising local add traffic fromcombining element 226 via fiber segment 302. The combined signal ispassed to OSC filter 298.

OSC filter 298 adds an OSC signal from the OSC interface 274, via theOSC sender 272 and fiber segment 284, to the combined optical signal andforward the combined signal as an egress transport signal to ring 204.The added OSC signal may be locally generated data or may be receivedOSC data passed through the EMS 290.

In the clockwise direction, OSC filter 300 receives an ingress opticalsignal from clockwise ring 202. OSC filter 300 filters the OSC signalfrom the optical signal and forwards the OSC signal to the OSC interface280 via fiber segment 286 and OSC receiver 278. OSC filter 300 alsoforwards the remaining transport optical signal to amplifier 242.

Amplifier 242 amplifies the signal and forwards the signal to ringswitch 246. Ring switch 246 is selectively operable to transmit theoptical signal to coupler 238 when the ring switch 246 is set to thethrough setting, or to transmit the optical signal to OSA connector 254when the ring switch 246 is set to the cross setting.

If the ring switch 246 is set in the cross position, the optical signalis not transmitted to couplers 238 and 236, the ring 204 is open at thenode 201, and dropping of traffic from the ring 202 does not occur atnode 201. However, adding of traffic to the ring 202 occurs at node 201.If the ring switch 246 is set in the through position, the opticalsignal is forwarded to couplers 238 and 236 and adding and dropping oftraffic to and from the ring 202 may occur at node 201.

Coupler 238 passively splits the signal from switch 246 into generallyidentical signals. A passthrough signal is forwarded to coupler 236while a drop signal is forwarded to distributing unit 224 via segment308. The signals may be substantially identical in content and/orenergy. Coupler 236 passively combines the passthrough signal fromcoupler 238 and an add signal comprising local add traffic fromcombining element 226 via fiber segment 306. The combined signal ispassed to OSC filter 296.

OSC filter 296 adds an OSC signal from the OSC interface 280, via theOSC sender 281 and fiber segment 288, to the combined optical signal andforwards the combined signal as an egress transport signal to ring 202.As previously described, the OSC signal may be locally generated data ordata passed through by EMS 290.

Prior to addition to the rings 202 and 204, locally-derived traffic istransmitted by a plurality of add optical senders 270 to combiningelement 226 of the node 201 where the signals are combined, amplified,and forwarded to the transport elements 220 and 222, as described above,via counterclockwise add segment 302 and clockwise add segment 306. Thelocally derived signals may be combined by the optical coupler 324, by amultiplexer or other suitable device.

Locally-destined traffic is dropped to distributing element 224 fromcounterclockwise drop segment 304 and clockwise drop segment 308.Distributing element 224 splits the drop signal comprising thelocally-destined traffic into multiple generally identical signals andforwards each signal to an optical receiver 268 via a drop lead 314. Thesignal received by optical receivers 268 may first be filtered byfilters 266. Filters 266 may be tunable filters or other suitablefilters and receivers 268 may be broadband or other suitable receivers.

EMS 290 monitors and/or controls all elements in the node 201. Inparticular, EMS 290 receives an OSC signal in an electrical format viaOSC filters 294, 296, 298 and 300, OSC receivers 276 and 278, OSCsenders 272 and 281, and OSC interfaces 274 and 280. EMS 290 may processthe signal, forward the signal and/or loopback the signal. Thus, forexample, the EMS 290 is operable to receive the electrical signal andresend the OSC signal to the next node, adding, if appropriate,node-specific error information or other suitable information to theOSC.

In one embodiment each element in a node 201 monitors itself andgenerates an alarm signal to the EMS 290 when a failure or other problemoccurs. For example, EMS 290 in node 201 may receive one or more ofvarious kinds of alarms from the elements and components in the node201: an amplifier loss-of-light (LOL) alarm, an amplifier equipmentalarm, an optical receiver equipment alarm, optical sender equipmentalarm, a distributing amplifier LOL alarm, a distributing amplifierequipment alarm, a combining amplifier LOL alarm, a combining amplifierequipment alarm, or other alarms. Some failures may produce multiplealarms. For example, a fiber cut may produce amplifier LOL alarms atadjacent nodes and also error alarms from the optical receivers.

In addition, the EMS 290 may monitor the wavelength and/or power of theoptical signal within the node 210 via connections (not shown) betweenthe OSA connectors 250, 254, 256, and 258 and an optical spectrumanalyzer (OSA) communicably connected to EMS 290.

The NMS 292 collects error information from all of the nodes 201 and isoperable to analyze the alarms and determine the type and/or location ofa failure. Based on the failure type and/or location, the NMS 292determines needed protection switching actions for the network 200. Theprotection switch actions may be carried out by NMS 292 by issuinginstructions to the EMS 290 in the nodes 201. After a failure is fixed,the network 200 does not require reverting. Thus, the open ring networkconfiguration does not change for protection switching, only thelocation of the openings. In this way, network operation is simplifiedand node programming and operation is cost minimized or reduced.

Error messages may indicate equipment failures that may be rectified byreplacing the failed equipment. For example, a failure of one of theamplifiers in the distributing element may trigger a distributingamplifier alarm. The failed amplifier can then be replaced. A failedcoupler in the distributing element may be likewise detected andreplaced. Similarly, a failure of an optical receiver or sender maytrigger an optical receiver equipment alarm or an optical senderequipment alarm, respectively, and the optical receiver or senderreplaced as necessary. The optical sender should have a shutter or coldstart mechanism. Upon replacement, no other switching or reversion froma switched state may be required. As described further below inreference to FIGS. 16 and 18, the NMS 292 may in response to certainmessages or combinations of messages trigger a protection switchingprotocol.

FIGS. 10A–C illustrate details of the node 201 and elements of the node201 in accordance with other embodiments of the present invention. Inthe embodiment of FIG. 10A, OSC signals are transmitted in-band withrevenue-generating traffic. In addition, redundant ring switches andvariable optical attenuators (VOAs) are provided in the transportelements. In the embodiment of FIG. 10B, the distributing element 224utilizes VOAs in the place of amplifiers. In the embodiment of FIG. 10C,the combining element 226 uses VOAs in place of amplifiers in order thatsignal levels to the clockwise ring via clockwise add fiber segment 306and to the counterclockwise ring via counterclockwise add fiber segment302 are able to be controlled independently of each other in order toadjust “through” signal levels. For example, the “through” signal levelof the clockwise and clockwise rings may differ from each other if thetransport elements do not have pre-amplifiers.

Referring to FIG. 10A, the node 350 comprises a distributing element 224and a combining element 226 as described above in reference to FIG. 9.In this embodiment, the combining element 226 may include two positionsafety switches 251 which may be opened to stop transmission of a signalonto a line with a fiber cut in order to allow the cut to be repairedsafely. It will be understood that the node 350 may include otherswitches or suitable devices to stop transmission of a signal onto afiber that is under repair. For example, the node 350 may include adevice in the transport elements to prevent ingress traffic on the ringfrom being transmitted out onto a span under repair.

The node 350 further comprises the counterclockwise add fiber segment302, counterclockwise drop fiber segment 304, clockwise add fibersegment 306, clockwise drop fiber segment 308, plurality of add leads312, plurality of drop leads 314, OSA connectors 250, 252, 256, and 258,inputs 248 and 254 and a plurality of passive connectors 230 asdescribed above in connection with FIG. 9.

Node 350 comprises a counterclockwise transport element 352 whichcomprises a passive optical splitter set including counterclockwise dropcoupler 232 and counterclockwise add coupler 234, and further comprisescounterclockwise ring switch 244. These elements are described above inreference to FIG. 9. Counterclockwise transport segment 352 furthercomprises redundant ring switch 382, OSC filter 372, and VOA unit 330.OSC filter 372 may comprise a thin film type or a fiber grating typefilter. Out-of-band OSC filters 216 and amplifier 214 are omitted inthis embodiment.

Node 352 further comprises a clockwise transport element 354 whichcomprises a passive optical splitter set including clockwise add coupler236 and clockwise drop coupler 238, and further comprises clockwise ringswitch 246. These elements are as described above in reference to FIG.9. Clockwise transport segment 354 further comprises redundant ringswitch 376, OSC filter 374 and VOA unit 330. OSC filter 374 may comprisea thin film type or a fiber grating type filter. The out-of-band OSCfilters 216 and amplifier 214 are omitted in this embodiment.

The VOA unit 330 is on an egress side of an isolator 332. The VOA unit330 includes a VOA 334, an optical splitter 336, a photodetector 338 anda controller 340. The isolator 332 prevents upstream feedback. The VOA334 attenuates the ingress signal to a specified power level based on afeedback loop including splitter 336 which taps the signal,photodetector 338 which detects the power level of the signal andfeedback controller 340 which controls VOA 334 based on the detectedpower level.

Node 352 further comprises a managing element 370 which comprises OSCsenders 272 and 281, OSC interfaces 274 and 280, OSC receivers 276 and278, and an EMS unit 290. These elements are as described above inreference to FIG. 9. Managing element 370 further comprises filters 360and 362, optical fiber segments 364 and 366, and OSC-add optical fibersegments 356 and 358.

In operation, OSC signals are transmitted in-band. OSC receivers 276 and278 are operable to receive ingress OSC signals via two of drop leads314. Filters 360 and 362 are operable to selectively filter the OSC datafrom the optical signals transmitted by distributing element 224. In oneembodiment, two wavelengths are dedicated to OSC signals: for example,1530.33 nm for clockwise-ring OSC signals, and 1531.12 nm forcounterclockwise-ring OSC signals. Filters 360 and 362 are tunedaccordingly. OSC filter 372 rejects the incoming OSC signal; 1531.12 nmand adds the OSC signal; 1531.12 nm via segment 356. OSC filter 374rejects the incoming OSC signal; 1530.33 nm and adds the OSC signal;1530.33 nm via segment 358. Processed OSC data may be added to the rings202 and 204 from clockwise OSC sender 281 and counterclockwise OSCsender 272 via counterclockwise OSC filter 372, counterclockwise OSCfilter 374 and OSC-add fiber segments 356 and 358, respectively.

The redundant ring switches 382 and 376 allow for continued circuitprotection in the event of switch failure and failed ring switches maybe replaced without interfering with node 350 operations orconfiguration. Redundant ring switches 382 and 376 may further compriseOSA connectors 378 and 386 to allow for monitoring of the wavelengthand/or power of the optical signal and inputs 380 and 384.

When the ring switch has the cross position, cascaded switchconfiguration allows switch operation test. Either switch 382 or 244 isallowed to take through or cross position for testing, because the otheris in cross position. When the ring switch is required to change fromthe through position to the cross position, the cascaded switchconfiguration gives redundancy to open the segment of the ring.Alternatively, redundancy in the event of a switch stuck in the closedposition can be accomplished without a redundant switch by turning offthe amplifier for that ring 202 or 204 in the node 201 with the failedswitch, thus effectively terminating the signal at the amplifier.

FIG. 10B illustrates the distributing unit 224 in accordance withanother embodiment of the present invention. In this embodiment, thedrop signals are attenuated or otherwise suitably controlled in thedistributing element 224 and VOA units, amplifiers or other suitableconditioners are not necessary in the transport elements 352 and 354.

Referring to FIG. 10B, distributing unit 224 includes two VOA units 330and a coupler 342 with a suitable number of egress drop leads 314. Aspreviously described, the VOA units 330 each attenuate an ingress signalto a specified power level through a feedback loop including splitter336, photodetector 338 and feedback controller 340.

FIG. 10C illustrates the combining element 226 in accordance withanother embodiment of the present invention. In this embodiment, thecombining element 226 includes two VOA units 330 for each add line 302and 306 to attenuate or otherwise suitably condition the add signals.The combining element 226 with VOA units 330 may be used in connectionwith distributing element 224 with VOA units 330 and, as previouslydescribed, the in-ring amplifiers, VOA units, or signal conditioners maybe omitted.

Referring to FIG. 10C, ingress signals are combined at a many to twooptical splitter 324 and passed to the VOA units 330. As previouslydescribed, the VOA units 330 each attenuate the add signal with VOA 334.VOA 334 is controlled by a feedback loop including optical splitter 336,photodetector 338 and feedback controller 340. In this way, add signalintensity may be suitably controlled for adding and transport in therings 202 and 204.

FIG. 11 illustrates the distributing element of node 201 in accordancewith another embodiment of the present invention. The embodiment shownin FIG. 11 may be used as an alternative to the distributing element 224of FIGS. 9 and 10.

Referring to FIG. 11, distributing element 390 comprises array waveguide gratings (AWGs) 392 and 394 operable to demultiplex the opticalsignal from coupler 310. Demultiplexing includes a filtering function,so that filters 266 are not required in this embodiment. Thedemultiplexed signals are forwarded to receivers 268 via leads 396.Thus, each receiver receives a discrete traffic channel rather than asignal comprising all of the traffic channels as with distributingelement 224.

FIG. 12 illustrates the combining element of a node 201 in accordancewith another embodiment of the present invention. The embodiment shownin FIG. 12 may be used as an alternative to the combining element 226 ofFIGS. 9 and 10.

Referring to FIG. 12, combining element 420 comprises coupler 400operable to receive signals from client via leads 402 and to combinethose signals into optical fibers 404 and 406. Amplifiers 408 and 410amplify the optical signals carried by fibers 404 and 406 respectively.Connectors 230 connect coupler 400 to optical fibers 404 and 406.Amplifiers 408 and 410 may comprise EDFAs or other suitable amplifiers.Combining element 420 further comprises couplers 422, 424, 426, and 428,leads 430 and 432, switches 434 and 436, and optical fibers 438 and 440.

In operation the combined signals from coupler 400 are amplified viaamplifiers 408 and 410 and split at couplers 422 and 426. One copy ofthe split signal is sent to couplers 424 and 428. Another copy of thesplit signal from couplers 422 and 426 is sent to switches 434 and 436respectively, via leads 432 and 430. Switches 434 and 436 are operableto selectively transmit an optical signal to couplers 424 and 428.

During normal operation, the switches 434 and 436 are open such thatonly one copy of local add traffic is provided to each transport element220 and 222 of node 201. In case of failure of amplifier 408, switch 434is switched to the on, or closed position, thus allowing for add trafficto be transmitted to both transport elements 220 and 222 through leads438 and 440. Likewise in case of failure of amplifier 410, switch 436 isswitched to the on, or closed position allowing for transmission of theadd traffic to both transport elements 220 and 222 through fibers 438and 440. In addition, the switching mechanism of the embodiment shown inFIG. 12 allows for replacement of the failed amplifier while stillallowing for transmission of the add traffic in the network 200.

FIG. 13 illustrates the optical network 200 with high level details ofthe nodes 206, 208, 210 and 212. As previously described, each nodeincludes a counterclockwise transport element 220, a clockwise transportelement 222, a distributing element 224, a combining element 226, and amanaging element 228. The transport elements add and/or drop traffic toand from the rings 202 and 204. The combining element 226 combinesingress local traffic to generate an add signal that is provided to thetransport elements 220 and 222 for transmission on the rings 202 and204. The distributing element 224 receives a dropped signal and recoverslocal egress traffic for transmission to local clients. The managingelement 228 monitors operation of the node 201 and/or network 200 andcommunicates with an NMS 292 for the network 200.

Referring to FIG. 13, each node 206, 208, 210 and 212 includes a ringswitch 214 in each transport element 220 and 222 that is controllable toselectively open or close the connected ring 202 or 204 prior to thedropping or adding of traffic by the transport element 220 or 222 in thenode. The ring switches 214 may be otherwise suitably positioned withinone or more or each node 201 prior to the dropping and/or adding oftraffic, at an inside or outside edge of the node 201 or between thenode and a neighboring node 201.

During normal operation, a single ring switch 214 is crossed orotherwise open in each ring 202 and 204 while the remaining ringswitches 214 are closed. Thus, each ring 202 and 204 is continuous orotherwise closed except at the ring switch 214 that is open. The ringswitches 214 that are open in the rings 202 and 204 together form aswitch set that effectively opens the rings 202 and 204 of the network200 in a same span and/or corresponding point of the network 200. A samespan is opened in the network 200 in that, for example, the nodes 201neighboring the span do not receive and/or receive for dropping ingresstraffic from the span. Such alignment of the open ring switches 214 in,along or at the periphery of a span allows each node 201 may communicatewith each other node 201 in the network 200 while avoiding or minimizinginterference from circulating traffic.

In the illustrated embodiment, ring switch 214 in the clockwisetransport element 222 of node 210 is crossed as is ring switch 214 inthe counterclockwise transport element 220 of node 208. The remainingring switches 214 are closed to a through position. A traffic channel500 added at node 210 travels around the rings 202 and 204 in exemplarylight paths 502 and 504. In particular, a counterclockwise light path502 extends from the combining element 226 of node 210 to thecounterclockwise transport element 220 where it is added tocounterclockwise ring 204. On counterclockwise ring 204, light path 502extends to node 208 where it is terminated by the crossed ring switch214 of the counterclockwise transport element 220. Clockwise light path504 extends from the combining element 226 of node 210 to the clockwisetransport element 222 of node 210 where it is added to clockwise ring202. On clockwise ring 202, light path 504 extends to ring 212, throughthe clockwise transport element 222 of ring 212, to ring 206, throughthe clockwise transport element 222 of ring 206, to node 208, throughthe clockwise transport element 222 of node 208, and back to node 210where it is terminated by the crossed ring switch 214 on the ingressside of the clockwise transport element 222. Thus, each node 206, 208,210 and 212 is reached by each other node from a single direction andtraffic is prevented from circulating around either ring 202 and 204 orotherwise causing interference.

FIG. 14 illustrates the optical network 200 with high level details ofthe nodes 206, 208, 210 and 212. The nodes each include thecounterclockwise and clockwise transport elements 220 and 222 as well asthe combining element 224, distributing element 226 and managing element228. In addition to adding and dropping traffic channels to and from therings 202 and 204, the transport elements 220 and 222 add and drop theOSC to and from the rings 202 and 204 for processing by managing element228.

Referring to FIG. 14, as previously described, the transport elements220 and 222 include an OSC filter 216 at an ingress point prior to thering switches 214 to filter out and/or otherwise remove the OSC from therings 202 and 204. In each node 201, the OSC signal from each ring 202and 204 is passed to corresponding optical receiver 276 and 278 of theOSC unit for processing by EMS 290. In addition, the OSC signalgenerated by the EMS 290 for each ring 202 and 204 is transmitted by theoptical sender 272 or 281 onto the corresponding ring 202 and 204 fortransmission to the next node 201.

In normal operation, each node 201 receives an OSC signal from theneighboring nodes along the rings 202 and 204, processes the signal andpasses the OSC signal on and/or adds its own OSC signal for transmissionto the neighboring nodes.

Placement of the OSC filters 216 at the periphery of the transportelements 220 and 220 outside the ring switches 214 allows each node 201to receive the OSC signal from its neighboring, or adjacent nodes 201regardless of the open/close status of its ring switches 214. If the OSCfilters are inside the ring switches 214, for example, in embodimentswhere the ring switches 214 are outside of the nodes 201, the OSCsignals may be looped back between rings 202 and 204 at the edges of theopen span. For example, for the illustrated embodiment, the EMS 290 ofnode 208 may pass received OSC information destined for node 210 fromthe clockwise OSC unit to the counterclockwise OSC unit for transmissionto node 210 on the counterclockwise ring 204. Similarly, OSC informationreceived at node 210 and destined for node 208 may be passed by the EMS290 of node 210 from the counterclockwise OSC unit to the clockwise OSCunit for transmission to node 208 on the clockwise ring 202.

FIG. 15 illustrates protection switching and light path protection fornetwork 200 in accordance with one embodiment of the present invention.As previously described, each node 206, 208, 210, and 212 includesclockwise and counterclockwise transport elements 220 and 222 as well asthe combining, distributing and managing elements 224, 226, and 228. Themanaging elements each communicate with NMS 292.

Referring to FIG. 15, a fiber cut 510 is shown in ring 204 between nodes206 and 212. In response, as described in more detail below, the NMS 292opens the ring switch 214 in counterclockwise transport element 220 ofnode 212 and the ring switch 214 in clockwise transport element 222 ofnode 206, thus effectively opening the span between nodes 206 and 212.After opening the rings 202 and 204 on each side of the break, the NMS292 closes any previously open ring switches 214 in the nodes 201.

After protection switching each node 201 in the network 200 continues toreceive traffic from each other node 201 in the network 200, and anoperable open ring configuration is maintained. For example, a signal512 originated in node 210 is transmitted on counterclockwise light path514 to nodes 208 and 206 and transmitted on clockwise light path 516 tonode 212. In one embodiment, the NMS 292, EMS 290 and the 2×2 ringswitches 214 may be configured for fast protection switching, with aswitching time of less than 10 milliseconds. In the other example, theinput monitor of ingress amplifier 242 on the clockwise ring 201 in thenode 206 detects the loss of light due to the fiber cut 510, then theEMS 290 in the node 206 may open the ring switch 214 in the node 206locally. The EMS 290 reports to NMS 292. The NMS opens the ring switch214 in the node 212 and closes any previous open ring switches 214 inthe nodes 201.

FIG. 16 is a flow diagram illustrating a method for protection switchingof an open ring optical network in accordance with one embodiment of thepresent invention. In this embodiment, the optical network may benetwork 200 including a plurality of nodes each having a ring switch ator proximate to an ingress point of each connected ring. The method maybe used in connection with other suitable network and nodeconfigurations.

Referring to FIG. 16, the method begins at step 550 with the detectionby the NMS 292 of a fiber cut of ring 202 or 204 of the network 200. TheNMS 292 may detect and locate the fiber cut based on the OSC and/orother signals communicated by the node EMSs 290 to the NMS 292. Forexample, a fiber cut may be detected by the NMS 292 based on an LOLalarm from a down stream preamplifier 242 of a neighboring node 201.

At step 552, the NMS 292 issues a command to the EMS 290 in the node 201immediately clockwise of the cut to open the clockwise ring switch 246in the clockwise transport element 222, this opening the clockwise ring202 at that node 201. The down stream preamplifier 242 may open theclockwise ring switch 246 on behalf of NMS 292 or EMS 290.

At step 554, the NMS 292 issues a command to the EMS 290 in the node 201immediately counterclockwise of the cut to open the counterclockwisering switch 244 in the counterclockwise transport element 220, thisopening the counterclockwise ring 204 at that node 201.

At step 556, any other ring switches 214 in the nodes 201 of the network200 are closed. Thus, each ring 202 and 204 is essentially continuouswith a single open point and/or segment. The open segment may be at adiscrete switch and/or transmission element or may include part, all oreven more than a span between nodes of the network 200. It will beunderstood that additional switches 214 in the rings 200 and/or 204 mayremain open and that amplifier, VOA and other suitable devices in therings 202 and/or 204 may be turned off so long as, in one embodiment,each node 201 is able to communicate with each other node 201 throughone of the rings 202 or 204.

An example of protection switching is illustrated by FIGS. 13 and 15.Referring back to FIG. 13, for example, the clockwise andcounterclockwise rings 202 and 204 of network 200 are open in thetransport elements 222 and 220 of nodes 210 and 208, respectively. Inresponse to at least a ring cut 510 as illustrated by FIG. 15,protection switching crosses ring switch 214 and clockwise transportelement 222 of node 206 and ring switch 214 of counterclockwisetransport element 220 of node 212. Thus, in FIG. 15 the clockwise andcounterclockwise rings 202 and 204 are opened at nodes 206 and 212,respectively. The previously crossed ring switches in nodes 208 and 210are closed to a through position to allow each node 201 in the network200 to continue to receive traffic from each other node 201 in thenetwork 200. The fiber cut 510 may be repaired at a convenient timeafter protection switching is completed. Furthermore, it should be notedthat, after repair of the fiber cut 510, there is no need to revert theswitches 214 and nodes 201 to their pre-cut states. For example, thenetwork initially configured as shown in FIG. 13 that is then configuredas shown in FIG. 15 due to fiber cut 510 may remain configured as shownin FIG. 15 even after the cut 510 has been repaired. In this way, thesteps shown in FIG. 16 may be repeated for any number of fiber cutevents.

As previously described, the ring switches 214 and the nodes 201 may bereconfigured to provide protection switching in response to other typesof network failures that would otherwise prevent one node 201 fromcommunicating local and/or other traffic to a neighboring node 201. Forexample, in response to failure of the preamplifier 242 of the clockwisetransport element 222 of node 206, the failed preamplifier 242 may beturned off and the adjacent ring switch 246 actuated from a closed, orthrough position to an open, or cross position. Failure of thepreamplifier unit 242 may be detected from a preamplifier equipmentalarm for that amplifier. As previously described, a crossed ring switch214 terminates traffic on the connected ring 202 or 204 but may pass thetraffic to the OSAs for monitoring by the EMS 290 and/or for loopbackand other types of testing. Next, the ring switch 214 of thecounterclockwise transport element 220 in node 212 may also berepositioned to the crossed position.

After the ring switches are crossed, the previously crossed ringswitches 214 are closed to a through position to allow each node 201 tofully communicate with each other node 201. During continued operation,the failed preamplifier unit 242 may be replaced and proper operation ofthe new preamplifier unit 242 confirmed with loopback and/or localizedtesting as described in more detail below. After the failed preamplifier242 is replaced and proper operation is confirmed, the network 200 maybe left in the current configuration, reverted to the previousconfiguration or configured to yet another configuration to supportlocalized and/or loopback testing within the network 200.

As another example, if the distributing amplifiers of node 206 report anLOL alarm and there is no alarm in the preamplifier 242 in the node 206but a LOL alarm from the preamplifier 242 in the nodes 208, the NMS 292may determine there is a failure of the drop coupler 238 in theclockwise transport element 222. In response, the NMS 292 may implementprotection switching as previously described for the preamplifierfailure with the failed coupler being replaced and tested. A failure ofthe ring switch 214 in the clockwise transport element 222 of node 206may be detected by an equipment alarm for that switch and protectionswitching implemented as previously described for the preamplifier unitfailure. In addition, if the switch 214 fails in a closed position, thepreamplifier 242 may be turned off to effectively open the ring at thepoint of the failed switch 214.

A failure of an amplifier in the combining element 226 may be detectedby an equipment alarm for a combining amplifier. For example, inresponse to an equipment alarm for a combining amplifier in thecombining element 226 of the clockwise transport element 222 of node210, the ring switch 246 of clockwise transport element 222 in node 212may be crossed and the ring switch 244 in the counterclockwise transportelement 220 of node 210 may also be crossed. Previously opened ringswitches 214 are then or at the same time closed and the failedcombining amplifier unit in node 210 replaced and tested to confirmproper operation. In another embodiment, where the combining element 226includes crossover protection switches as illustrated by FIG. 12, theswitch 434 or 436 for the nonfailed amplifier may be closed to allow theworking amplifier to transmit traffic both directions on the rings 202and 204. In this embodiment, failure of a combining amplifier isprotected without affecting the network configuration.

FIG. 17 illustrates OSC protection for network 200 in response to a linecut in accordance with one embodiment of the present invention. In thisembodiment, optical-electrical loopback in the managing elements 228 ofthe nodes 201 is used for protection of OSC.

Referring to FIG. 17, a fiber cut or other line break 580 is shown inclockwise ring 202 between nodes 206 and 212. In response to the fibercut 580, an optical-electrical loopback 582 is established from thecounterclockwise OSC system to the clockwise OSC system through EMS 290in node 206 and from the clockwise OSC system to the counterclockwiseOSC system through EMS 290 in node 212.

In a specific embodiment, the optical-electrical loopback in node 206comprises receiving at the counterclockwise OSC unit of the managingelement 228 of node 206 the OSC 584 from the counterclockwise ring 204and processing the OSC at the EMS 290 as described above in reference toFIG. 9. However, instead of transmitting the processed OSC as an egresssignal on the counterclockwise ring 204 from node 206, the processed OSCis transmitted from the EMS 290 to the clockwise OSC unit and then ontoclockwise ring 202, therefore looping the OSC back at node 206 from acounterclockwise to a clockwise signal.

Similarly, the optical-electrical loopback in node 212 comprisesreceiving at the clockwise OSC unit of the management element 228 ofnode 212 the OSC 586 from the clockwise ring 202 and processing the OSCat the EMS 290 as described above in reference to FIG. 9. However,instead of transmitting the processed OSC as an egress signal on theclockwise ring 202 from node 212, the processed OSC is transmitted fromthe EMS 290 to the counterclockwise OSC unit and then tocounterclockwise ring 204, therefore looping the OSC back at node 212from a clockwise to a counterclockwise signal. In this way, each node201 in the network 200 continues to receive the OSC from each other node201 in the network 200. The optical-electrical loopback 582 may be usedduring normal operation and may be used when the OSC signal istransmitted in-band or other embodiment which the OSC signal passesthrough the ring switches 214. For example, in FIG. 14, if a ring switch214 in the counterclockwise transport element 220 of the node 208 and aring switch 214 in the clockwise element 222 have cross positions shownin FIG. 13, optical-electrical loopbacks may be deployed from clockwiseto counterclockwise in the node 208 and from counterclockwise toclockwise in the node 220. In this embodiment, the OSC flow procedure isthe same in both the normal and the protection (fiber cut). Thus,programming and control of the network 200 is simplified.

FIG. 18 illustrates a method for OSC protection switching in an opticalnetwork in accordance with one embodiment of the present invention. Inthis embodiment, protection switching is implemented in response to afiber cut. However, it will be understood that OSC protection switchingmay be implemented in response to other types of failures and may beimplemented in conjunction with light path protection switching.

Referring to FIG. 18, the method begins at step 600 with the detectionby the NMS 292 of a fiber cut 580 in a span of a ring 202 or 204 of theoptical network 200. The NMS 292 may detect the failure based on OSCand/or other signals from EMS 290 of the nodes 201.

At step 602, the NMS 292 issues a command to the EMS 290 in the node 201immediately clockwise of the cut 580 to form an electrical loopback fromthe counterclockwise OSC unit to the clockwise OSC unit, thus creating,as described above, an optical-electrical loopback of the OSC from thecounterclockwise ring 204 to the clockwise ring 202. The EMS 290 in thenode 206 may detect the fiber cut 580 and execute this electricalloopback without the command from NMS 292.

At step 604, the NMS 292 issues a command to the EMS 290 in the node 201immediately counterclockwise of the cut to form an electrical loopbackfrom the clockwise OSC unit to the counterclockwise OSC unit, thuscreating, as described above, an optical-electrical loopback of the OSCfrom the clockwise ring 202 to the counterclockwise ring 204. It will beunderstood that in this and other forms of protection switching, the NMS292 may itself directly control devices in the nodes 201, may otherwisecommunicate with the devices to provide protection switching and/or themanaging elements 228 of the nodes 201 may communicate among themselvesto provide the functionality of the NMS 292.

At step 606, any other nodes 201 containing loopbacks that may have beenpreviously formed are reverted to a non-loopbacked state. If the OSCoptical-electrical loopback procedure is deployed in nodes which havethe ring switch with cross position, the reverting is not required. Inthis way, OSC data may continue to be transmitted by and received andprocessed at each node 201 in the network 200. After completion of themethod, the fiber cut 580 may be repaired and tested. Also as above,after repair of the fiber cut 580, there is no need to revert thenetwork 200 to its pre-switch state.

FIG. 19 illustrates OSC protection switching in the network 200 inresponse to an OSC equipment failure in accordance with one embodimentof the present invention. In this embodiment, protection switching isimplemented for failure of an OSC sender. Failure of an OSC filter 216or an OSC receiver 276 or 278 may necessitate similar protectionswitching so that each node 201 may continue to be serviced by OSC dataeven in the event of an equipment failure.

Referring to FIG. 19, counterclockwise OSC sender 281 of node 206 isdetected as having failed. In a particular exemplary embodiment, afailure of an OSC optical sender 272 or 281 or an OSC optical receiver276 or 278 may be detected by the NMS 292 based on an LOL alarm for theoptical receiver or a downstream optical receiver with or withoutanother failure alarm. For example, an equipment alarm for the opticalsender 281 in the counterclockwise OSC unit of the managing element 282of node 206 would indicate a failure 610 of that optical sender. Inresponse, the NMS 292 or EMS 290 in the node 206 may loopbackcounterclockwise OSC 612 to clockwise OSC at node 206. At node 212, theNMS 292 loopbacks the clockwise OSC 614 to the counterclockwise OSC. Anyprevious loopbacks in nodes 208 and/or 210 are broken and theinformation sent through the nodes.

After protection switching, the failed optical sender 281 may bereplaced and thereafter tested using clockwise OSC. After confirmingoperation of the replaced optical sender 281, the network 200 maycontinue to operate in its present state or may revert to the initialOSC state. As previously discussed, for a fiber cut between nodes 206and 210 the same procedure may be followed with the fiber cut repairedand tested.

FIGS. 20–22 illustrate loopback and localized testing in optical network200 in accordance with various embodiments of the present invention.Using the loopback and localized testing, sections of the rings 202 and204 and devices within those sections may be tested to determine faultsand failures and/or to confirm proper operation of equipment.

FIG. 20 illustrates optical loopback testing of a light path in theoptical network 200 in accordance with one embodiment of the presentinvention. In this embodiment, the ring switches 214 are each 2×2switches connectable to OSAs through connections 250 and 254. A testsignal may be fed into the networks 202 and 204 through 248 and 252 ofthe 2×2 switch with cross position. The OSAs may analyze receivedsignals and communicate the signals or information about the signals toEMS 290 and/or NMS 292.

Referring to FIG. 20, the network 200 is operating with thecounterclockwise ring switch 246 crossed in node 210 and thecounterclockwise ring switch 244 crossed in node 208. In thisembodiment, test signal 620 may be inserted into the network 200 at thecombining element 226 of node 210 for transmission on light path 622 ofthe clockwise ring 202 and on light path 624 of counterclockwise ring204. In another embodiment, the test signal may be inserted from theport 256 of the 2×2 switch 240. Thus, testing may be performed in and/orfrom a node with no transmitters.

In particular, light path 624 is added to the counterclockwise ring 204by counterclockwise transport element 220. On the counterclockwise ring204, the light path 624 is transmitted to node 208 where it isterminated at crossed ring switch 214 of the counterclockwise transportelement 220. In the clockwise direction, light path 622 is added to theclockwise ring 202 by clockwise transport element 222. On the clockwisering 202, light path 622 is transmitted to and through nodes 212, 206and 208 before returning to node 210. At node 210, light path 622 onclockwise ring 204 is terminated on the ring by ring switch 214 of theclockwise transport element 222 but is passed to an OSA via connector254. The OSA 254 analyzes the received signal and passes the resultsand/or the signal to the managing element 228. Thus, the light path ofring 204 from node 210 to node 212, to node 206, and to node 208 may betested. In a similar fashion, other light paths may be tested by openinga ring switch 214 at other nodes 201 such that for example, thereturning signal is passed through a crossed switch to an OSA foranalysis.

FIG. 21 illustrates loopback testing of a light path in the opticalnetwork 200 in accordance with another embodiment of the presentinvention. In this embodiment, the ring switches 214 may be two-wayswitches as well as 2×2 switches.

Referring to FIG. 21, the clockwise ring 202 is opened at node 210between the clockwise couplers 70. The opening 630, corresponding to 230in FIG. 9, may be accomplished by physically separating the opticalfiber at a point between the clockwise couplers 70, with a two positionswitch or by any other methods to open the ring 202. As a result, thering switch 214 in the clockwise transport element 222 of node 210 mayremain in a closed, or through position as traffic is terminated by theopening 630.

Test signal 632 is added to the network 200 at combining element 226 ofnode 210 for transmission on light path 634 of the clockwise ring 202and on light path 636 of counterclockwise ring 204. In particular, lightpath 636 is added to counterclockwise ring 204 by counterclockwisetransport element 220. On the counterclockwise ring 204, light path 636is transmitted to node 208 where it is terminated by ring switch 214 ofthe counterclockwise transport element 220. In the clockwise direction,light path 634 is added to the clockwise ring 202 by clockwise transportelement 222. On the clockwise ring 202, light path 634 is transmitted toand through nodes 212, 206 and 208 before returning to node 210. At node210, the ring switch 214 of clockwise transport element 222 is closed toallow ingress traffic to proceed to the drop coupler 70 before beingterminated on the ring 202 by the opening 630. Because the opening 630of the ring 202 occurs at a point after the ingress signal in node 210has been dropped by the drop coupler 70 of the clockwise transportelement 222 of node 210, the lightpath 634, unlike the example shown inFIG. 20, is received by the distributing element 224 and may be passedto EMS 290 and/or NMS 292. In a similar fashion, other light paths maybe tested by opening a ring 202 or 204 between the couplers 70 of othernodes 201.

FIG. 22 illustrates localized area testing in the optical network 200 inaccordance with one embodiment of the present invention. Thisembodiment, the 2×2 switches and the OSAs are used to route and monitortest signals.

Referring to FIG. 22, a localized area 640 may be defined as necessaryfor light path or component testing, repair, or replacement. In theillustrated embodiment, the localized area 640 extends from a portion ofthe combining and transport elements 220, 222, 226 and 228 of node 210across the clockwise and counterclockwise rings 202 and 204 to thedistributing 224 and managing elements 228 of node 208 as well as aportion of the transport and combining elements 220, 222 and 226 of node208. To isolate the elements of the localized area 640 from the rest ofthe in-service network, the clockwise ring switch 214 of node 210 andthe counterclockwise ring switch 214 of node 208 are opened. Thelocalized area thus includes the opposite parts of two neighboring nodessuch that, in one embodiment, a localized area 640 may be definedcovering any device of any node in the network 200.

The localized area 640 may be tested with a first light path 642 addedto the network 200 by the combining element 226 of node 210. Light path642 is added to the counterclockwise ring 204 by the counterclockwisetransport element 220 and travels on the counterclockwise ring 204 tonode 208 where it is dropped from the ring by counterclockwise ringswitch 214 to an attached OSA and thus to the managing element 228 inthe node 208. Conversely, a second light path 644 is added to thenetwork 200 by combining element 226 of node 208 and added to theclockwise ring 202 by the clockwise transport element 222. Light path644 travels to node 210 on the clockwise ring 204 and is terminated fromthe ring by clockwise ring switch 214 which drops the light path to aconnected OSA and thus to the managing element 228 in the node 210.Thus, testing of replacement or repair components within localized area640 may be conducted without interfering with the in-service network.

FIG. 23 illustrates a method for inserting a node 201 into the opticalnetwork 200 in accordance with one embodiment of the present invention.Node insertion may take full advantage of the scalability in the designof network 200. Other suitable elements may be similarly insertedbetween the existing nodes 201 of the optical network 200.

Referring to FIG. 23, the method begins with step 650 wherein theclockwise ring switch 214 is opened in the node 201 immediatelyclockwise of the insertion point for the new node. Proceeding to step652, the counterclockwise ring switch 214 is opened in the node 201immediately counterclockwise of the insertion point. At step 654, anyother open ring switches 214 are closed. Thus, the nodes 201 of thenetwork 200 may each communicate with each other without communicatingacross a span in which the new node is to be added.

Proceeding to step 656, the new node is inserted at the insertion point.Such insertion may require the physical separation of the clockwise andcounterclockwise optical ring fibers. At step 658, the operation ofamplifiers, switches, and other elements of the new node may be checkedand tested.

Proceeding to step 660, the counterclockwise switch 214 in the new nodeis opened. At step 662, the counterclockwise switch 214 is closed in thenode 201 immediately counterclockwise of the new node. In this way, thecounterclockwise ring 204 is open at the new node and the clockwise ring202 is open at the node 201 immediately clockwise of the new node. Inanother embodiment, the clockwise switch 214 in the new node may beopened and the clockwise switch 214 in the node immediately clockwise ofthe new node closed.

FIG. 24 illustrates an optical network 700 with a combination of activenodes 701 and passive nodes 702 connected by clockwise andcounterclockwise optical rings 704 and 706 in accordance with oneembodiment of the present invention. In this embodiment, the passivenodes 702 may be passive in that they include no switches and/or noswitches connected in the optical rings 704 and/or 706 while the activenodes 701 may be the nodes 201 of network 200 or other nodes includingoptical switches 214 in the transport elements or otherwise connectedwithin the optical rings 704 or 706.

The passive nodes 702 may be of a simpler and less expensive design. Inthis way, the addition of the passive node 702 may allow for additionaladd/drop nodes in the network 700 while minimizing the additional costsassociated with the additional nodes. Because the nodes 702 are passive,however, they are not included in protection switching and may beisolated when the ring switches 214 in the neighboring nodes are crossedor otherwise open. Thus, in one embodiment, the passive nodes 702 may beused for low priority traffic. The passive nodes 702, as well as othernodes, may be add only nodes, drop only nodes or add/drop nodes.

FIG. 25 illustrates details of the passive node 702 of the opticalnetwork 700 in accordance with one embodiment of the present invention.In this embodiment, the passive node 702 includes an integrated add/dropcoupler.

Referring to FIG. 25, the node 702 comprises a counterclockwisetransport element 714, a clockwise transport element 716, a distributingelement 718, and a combining element 720. The transport element 714 and716 and the distributing and combining element 718 and 720 comprisepassive couplers 70. The passive coupler 70 of the transport elementsare 2×2 splitters. Isolators in the ingress side of the transportelements may avoid the interference due to multiple-reflection. Thepassive couplers 70 of the distributing and combining elements 718 and720 are 2×4 splitters. The output leads of the coupler 70 of thedistributing element feeds into filters 266 and receiver 268, describedpreviously in reference to FIG. 9. Transmitters 270, also described inreference to FIG. 9, feed into the coupler 70 of the combining element720. Being comprised, in one embodiment, of entirely passive components,the passive node 702 provides for flexible add/drop capability whilebeing simple and relatively inexpensive.

FIG. 26 is a block diagram illustrating details of a passive node of thenetwork 700 in accordance with another embodiment of the invention. Inthis embodiment, node 702 uses discreet add and drop couplers.

Referring to FIG. 26, the passive node 730 comprises distributingelement 718, combining element 720, filter 266, receiver 268, andtransmitter 270 as described above in reference to FIG. 25. However,counterclockwise transport element 732 and clockwise transport element734 each comprise a pair of transport couplers in addition to isolators.In this embodiment, the splitting ratio of the drop coupler isindependently determined from the splitting ratio of the add coupler. Asdescribed above in reference to FIG. 4, an arrangement wherein thetransport elements comprise a plurality of couplers each having one or aportion of the combiner or splitter elements may reduce channelinterference by dropping local traffic from a ring 704 or 706 beforeadding the local traffic to the rings.

Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. An optical network, comprising: a plurality of nodes; a first ringand a second ring connecting the nodes, the first ring operable totransport traffic in a first direction and the second ring operable totransport traffic in a second direction; the nodes each including afirst transport element coupled to the first ring and a second transportelement coupled to the second ring; the transport elements eachcomprising an optical splitter unit and an optical switch; the opticalsplitter unit operable to add local traffic to the coupled ring and todrop local traffic from the coupled ring; and the optical switchoperable to open the associated ring and direct ingress traffic from theassociated ring to a monitoring element for testing of an includedsignal.
 2. The optical network of claim 1, the optical switch comprisinga 2×2 switch operable in a closed position to pass ingress traffic tothe optical splitter unit and in a crossed position to direct theingress traffic to the monitoring element.
 3. The optical network ofclaim 1, the monitoring unit comprising a optical spectrum analyzer(OSA).
 4. The optical network of claim 1, the monitoring unit comprisingan element management system.
 5. The optical network of claim 1, whereina corresponding set of the optical switches in disparate nodes iscrossed to open each ring at at least one point and a signal added tothe ring in a transport element comprising one of the crossed switchescirculates around the ring and is dropped from the ring by the crossedswitch to the monitoring element for testing of the signal.
 6. A methodfor testing a signal path in an open ring network, comprising: openingeach of a plurality of rings transporting traffic in disparatedirections at at least one point with a switch; at an open switchdirecting traffic from a connected ring to a monitoring element; and atthe monitoring element, testing a signal received from the ring.
 7. Themethod of claim 6, wherein the signal is transmitted from a nodeincluding the monitoring element around the connected ring back to thenode before testing.
 8. The method of claim 6, wherein the monitoringelement comprises an optical spectrum analyzer (OSA).
 9. The method ofclaim 6, wherein the monitoring element comprises an element managementsystem.
 10. The method of claim 6, wherein the switch comprises a 2×2switch including a closed position operable to forward traffic along theconnected ring.
 11. A system for testing a signal path in an open ringnetwork, comprising: means for opening each of a plurality of ringstransporting traffic in disparate directions at at least one point;means at the opening for directing traffic from a connected ring to amonitoring element; and means at the monitoring element for testing asignal received from the ring.
 12. The system of claim 11, wherein thesignal is transmitted from a node including the monitoring elementaround the connected ring back to the node before testing.
 13. Thesystem of claim 11, wherein the monitoring element comprises an opticalspectrum analyzer (OSA).
 14. The system of claim 11, wherein themonitoring element comprises an element management system.
 15. Thesystem of claim 11, wherein the means for opening the ring comprises a2×2 switch including a closed position operable to forward traffic alongthe connected ring.
 16. An optical network, comprising: a plurality ofnodes; a first ring and a second ring connecting the nodes, the firstring operable to transport traffic in a first direction and the secondring operable to transport traffic in a second, disparate direction; thefirst ring open at a first optical ring switch in a first node and thesecond ring open at a second optical ring switch in a second node; alight path extending on one of the rings from an add point in a nodeincluding the opening of the ring, around the ring and back to a droppoint in the node; and a monitoring element testing a signal transmittedin the light path.
 17. The optical network of claim 16, wherein the droppoint precedes the opening of the ring in the node.
 18. The opticalnetwork of claim 16, wherein the drop point is at the open point of thering in the node.
 19. The optical network of claim 16, wherein thesignal is dropped from the ring in the node along with local traffic.20. The optical network of claim 19, wherein the signal and the localtraffic are dropped by a passive optical splitter.
 21. The opticalnetwork of claim 20, the node further comprising an optical spectrumanalyzer (OSA) for testing the signal.
 22. The optical network of claim16, wherein the signal is dropped by one of the optical ring switches.23. A method for testing a localized area of an open ring network duringnormal operation of the network, comprising; opening a first ring at afirst point and a second ring of the network at a second point whilemaintaining normal operation of the network with full communicationbetween nodes collectively on the first and second rings, wherein thefirst and second rings are operable to transport traffic in disparatedirections, the open points of the rings are in neighboring nodes andeach open point is operable to terminate traffic on a connected ringfrom the neighboring node; in a first of the neighboring nodes,transmitting a signal onto a ring transporting traffic directly to thesecond of the neighboring nodes; in the second of the neighboring nodes,dropping traffic prior to the open point from the ring to a managingelement; and testing the signal at the managing element.
 24. The methodof claim 23, wherein the monitoring element comprises an opticalspectrum analyzer (OSA).
 25. The method of claim 23, wherein themonitoring element comprises an element management system.
 26. Themethod of claim 23, wherein at least one of the rings is opened by aphysical separation of a fiber of the ring.
 27. The method of claim 23,wherein at least one of the rings is opened with a switch.
 28. Themethod of claim 27, wherein the switch comprises a 2×2 switch operablein a crossed position to open the ring and to direct traffic from thering to the monitoring element.
 29. A system for testing a localizedarea of an open ring network during normal operation of the network,comprising; means for opening a first ring at a first point and a secondring of the network at a second point while maintaining normal operationof the network with full communication between nodes collectively on thefirst and second rings, wherein the first and second rings are operableto transport traffic in disparate directions, the open points of therings are in neighboring nodes and each open point is operable toterminate traffic on a connected ring from the neighboring node; meansfor in a first of the neighboring nodes, transmitting a signal onto aring transporting traffic directly to the second of the neighboringnodes; means for in the second of the neighboring nodes, droppingtraffic prior to the open point from the ring to a managing element; andmeans for testing the signal at the managing element.
 30. The system ofclaim 29, wherein the monitoring element comprises an optical spectrumanalyzer (OSA).
 31. The system of claim 29, wherein the monitoringelement comprises an element management system.
 32. The system of claim29, wherein at least one of the rings is opened by a physical separationof a fiber of the ring.
 33. The system of claim 29, wherein at least oneof the rings is opened with a switch.
 34. The system of claim 33,wherein the switch comprises a 2×2 switch operable in a crossed positionto open the ring and to direct traffic from the ring to the monitoringelement.